At geological time scales, what really controls the climate isn’t the atmosphere, it’s the ground. Most of Earth’s carbon dioxide is held underground, in reservoirs of natural gas and oil, but also in the rocks themselves. As the planet’s tectonic plates slide and churn against one another, they bury carbon deep beneath the surface while exposing fresh rock that will soak up more carbon over time.
That carbon can be liberated in large volcanic events, causing mass extinctions. But the process can also work the other way, where rocks pull carbon from the sky. A new study from MIT researchers claims that Earth’s last three major ice ages were caused by collisions of tectonic plates bringing fresh, carbon-hungry rock to the surface. Over millions of years, these rocks sucked up enough carbon dioxide from the atmosphere to cause temperatures to plummet and send glaciers marching outward from the poles.
The process is simple. Much of the rock in Earth’s mantle is composed largely of silicate, and when exposed to the air, it will naturally react with carbon dioxide, forming new minerals that sequester carbon as a solid. This process is much more likely to occur in the tropics where temperatures are higher and frequent rain will wash soils away to expose bare rock.
At certain points in Earth’s history, oceanic tectonic plates in the tropics have collided with continental plates, sliding over the top of them and exposing hundreds of thousands of square miles of fresh rock to the air. These pile-ups, called arc-continent collisions, create a generous supply of fresh rock. Weathering processes begin as they come into contact with air and over the course of a few million years, carbon is gradually drained from the atmosphere.
Direct link to the study: http://science.sciencemag.org/content/early/2019/03/13/science.aav5300 [sciencemag.org]